The Bric-à-Brac BTB/POZ transcription factors are necessary in niche cells for germline stem cells establishment and homeostasis through control of BMP/DPP signaling in the Drosophila melanogaster ovary.

Author summary A stem cell is able to divide and produce two daughter cells, one of which retains stem cell status, the second, in contrast, becoming specialized for functions in the corresponding organ. Harnessing the potential of stem cells therefore represents opportunities for cell therapy upon organ damage. Maintenance of stem cells requires their interaction with specific cellular microenvironments called niches. Very little is known about how stem cell microenvironments are established during development and in pathological settings such as during cancer metastasis. The Drosophila adult ovary is probably the best characterized niche-stem cell system. It has been shown that niche cells secrete signaling proteins of the Bone Morphogenetic Protein (BMP) family, which by binding to receptors present at the membrane of adjacent Germline Stem Cells (GSCs), instruct these cells to maintain stem cell status. The analysis of ovaries mutant for the two bric-a-brac (bab) genes, showed that they are necessary within precursor cells for correct formation of GSC niches and for activation of the BMP pathway leading to the establishment of the first GSCs in the developing ovary. We have also found that Bab proteins contribute to GSC maintenance in the adult along with other transcription factors, Engrailed/Invected. Importantly, overproduction of one of the Bab proteins leads to the production of GSC tumors in the adult ovary underlining a role for this protein in GSC homeostasis. Many studies have focused on the mechanisms of stem cell maintenance via their interaction with a particular niche or microenvironment in adult tissues, but how formation of a functional niche is initiated, including how stem cells within a niche are established, is less well understood. Adult Drosophila melanogaster ovary Germline Stem Cell (GSC) niches are comprised of somatic cells forming a stack called a Terminal Filament (TF) and associated Cap and Escort Cells (CCs and ECs, respectively), which are in direct contact with GSCs. In the adult ovary, the transcription factor Engrailed is specifically expressed in niche cells where it directly controls expression of the decapentaplegic (dpp) gene encoding a member of the Bone Morphogenetic Protein (BMP) family of secreted signaling molecules, which are key factors for GSC maintenance. In larval ovaries, in response to BMP signaling from newly formed niches, adjacent primordial germ cells become GSCs. The bric-a-brac paralogs (bab1 and bab2) encode BTB/POZ domain-containing transcription factors that are expressed in developing niches of larval ovaries. We show here that their functions are necessary specifically within precursor cells for TF formation during these stages. We also identify a new function for Bab1 and Bab2 within developing niches for GSC establishment in the larval ovary and for robust GSC maintenance in the adult. Moreover, we show that the presence of Bab proteins in niche cells is necessary for activation of transgenes reporting dpp expression as of larval stages in otherwise correctly specified Cap Cells, independently of Engrailed and its paralog Invected (En/Inv). Moreover, strong reduction of engrailed/invected expression during larval stages does not impair TF formation and only partially reduces GSC numbers. In the adult ovary, Bab proteins are also required for dpp reporter expression in CCs. Finally, when bab2 was overexpressed at this stage in somatic cells outside of the niche, there were no detectable levels of ectopic En/Inv, but ectopic expression of a dpp transgene was found in these cells and BMP signaling activation was induced in adjacent germ cells, which produced GSC-like tumors. Together, these results indicate that Bab transcription factors are positive regulators of BMP signaling in niche cells for establishment and homeostasis of GSCs in the Drosophila ovary.